In the decade of the 1970's, criteria for nuclear power production facilities were the subject of re-evaluation by a variety of entities. During that interval, electrical load growth slowed, the accident at Three Mile Island occurred, and the development of nuclear power facilities using then existing design criteria diminished correspondingly. The power production industry then perceived a need for simpler and safer nuclear plants with reduced plant capital and operating costs and improved plant availability and reliability. By the end of that decade, an international advanced engineering team (AET) was assembled and called upon to evaluate worldwide boiling water reactor (BWR) technology and set forth the design aspects for a BWR which would combine the best of global design features and technologies. This effort established the present day basic advanced boiling water reactor (ABWR) design parameters.
Anticipated as a standard design for the decade of the 1990's, the ABWR utilizes a reactor vessel (RPV) having an internal diameter of 7.1 m and height of 21 m. Such diametric vessel sizing is currently considered to be a maximum value in consonance with the practicalities of its manufacture. Reactor maintenance and operation are improved with the ABWR through the use of internal circulation pumps in place of the external pumps of most currently operating plants. This recirculation feature eliminates piping, decreases construction time, and reduces in-service inspections. Such internal circulation pumps also serve to enhance safety by eliminating large reactor vessel nozzles and piping below the top of the core. As a result, the fuel remains covered with water even in the case of a postulated loss-of-coolant. This annular space between the RPV shroud and the vessel wall has been sized to permit positioning and servicing of the peripherally disposed internal pumps. This, in turn, has lead to a core structure with an active fuel length (AFL) of 3.81 m (about 12 feet), 872 fuel assemblies and 205 cruciform control rods, all contained within a core shroud with an inside diameter of 5.486 m. These cruciform control rods may be withdrawn by a control rod drive into and out of cylindrical guide tubes mounted within a lower plenum. Thus configured, the ABWR is intended to achieve a gross thermal power of 3926 MWt and a net electrical output of 1356 MWe and is highly regarded by the power industry.
With the establishment of initial ABWR power plants now under way, investigators in this field are contemplating improvements to the ABWR for the next decade. Designs for this next generation of the ABWR look to concerns not only for cost per kilowatt and power performance improvements, but also to achieving improved safety employing passive features with extended operator response time requirements, for example, from 30 minutes to 3 days. Such safety criteria press for a lower density core which, in turn, necessitates a larger core volume to maintain desired power output. However, the cost and manufacturing based constraints of restricted vessel diameter remain.
An enhancement of the safety aspects of plant performance also will look to improved stability of performance. One consideration for safety contemplates an all pumps stop condition (all pumps trip and rapid revolution coast down case) where forced circulation of water through the core rapidly diminishes As the core flow diminishes, the power-to-flow ratio of the system may actually increase with the possibility of the core exhibiting a thermal-hydraulic-reactivity coupled instability. Under such conditions, flow oscillation may occur within an individual fuel bundle of a fuel assembly, which, in turn, will result in oscillation of the void or steam content within the bundle and that, in turn, will result in a neutron flux oscillation, attendant power oscillation, and dampened heat flux oscillations. The latter damping occurs due to the time constant necessarily occurring between the generation of power in the fuel and increased temperatures at its clad. In the ABWR, the region of the power flow map where this condition of instability may occur is termed an "Instability Exclusion Region" and current procedure mandates automatic insertion of control rods at its location. It is, of course, desirable to eliminate this phenomenon, a task which may be accomplished by resort to lower power densities and shortened active fuel length (AFL). The latter fuel length reduction approach stems from an observation that the thermal-hydraulic instability is related to the boiling length and two phase pressure drop losses of the fuel. Stability ensues with the shortening of AFL. Notwithstanding the above proposed solutions, all must fall within the rigid constraints of maintaining the established diameter of the reactor vessel (RPV).